1. Field of the Invention
The present invention relates to intrusion detection systems and, specifically to a system using bistatic radar and Doppler radar processing techniques in conjunction with phase-coded pulse compression methods to achieve a high resolution detection zone "window".
The term "pulse compression" is used in the sense given in the I.E.E.E. Standard Dictionary, namely: "The coding and processing of a signal pulse of long time duration to one of short time duration and high range resolution, while maintaining the benefits of high pulse energy."
2. Description of Related Art
Intrusion detection systems can be of a line sensor type or volumetric sensor type. Line sensors provide perimeter security. In such systems, targets passing between the antennas cause partial or complete blockage of the transmit signal, resulting in the declaration of an alarm. Such systems cannot, however, provide true Doppler detection since there is no net radial movement. Leaky cables have also been used to provide perimeter coverage but are difficult to deploy rapidly. The operation of a leaky cable sensor above ground is troubled by moving foliage, sunlight, temperature drift and moisture around and on the cables. The result can be a high false alarm rate and an unsatisfactory detection performance.
The present invention relates to a volumetric sensor. There is a requirement for such a volumetric security sensor to detect intruders and vehicles. For example, weapons stockpiles, mobile C.sup.3 I resources and garrisons must be alerted to an intrusion well before the intruder can wreak havoc on the resource. The requirement typically is for a detection zone which is at least a few tens of meters away from the resource to allow unrestricted movement about the resource itself. It may be impractical to position the intrusion detection system in or near the detection zone. Consequently, there is a requirement for a security sensor that can provide a detection zone far enough away from the resource and the sensor itself to allow security forces to react. A well-defined zone centered 20-200 m away from the resource is sufficient for a number of security applications. The security sensor should be able to detect and track intrusions to allow security forces to quickly locate and intercept the potential intruder.
Any intrusion detection radar system should have a well defined detection zone. Movement outside the detection zone should not result in an alarm. Personnel movement in the vicinity of the antennas, such as within an encampment, must be tolerated by the system. Since the detection zone is usually some distance from the antennas, radar range-gating techniques are necessary.
The sensor must provide detection of a variety of ground level intrusions, such as vehicles and both a crawling and on-foot intruder. Airborne intrusion (e.g., hang glider, parachute) must also be detected by the radar system. The detection process for such targets can be optimized by using coherent or synchronous detection techniques, thus enabling Doppler signal processing. This allows both magnitude and phase information of the returned signal to be used in processing the data.
Foliage penetration capabilities are required for a number of security applications. The sensor must also be able to discriminate between the signal returned from blowing foliage and a legitimate target. Signal processing techniques, for example Fourier analysis and Kalmus filtering, are commonly used to help "unmask" an intruder's scattered signal from the clutter return signal. The sensor must be able to maintain a high detection capability/low false alarm rate over all weather conditions.
There are a variety of techniques that can be used to determine the range of the received signal. The traditional means to achieve this has been pulse-type radar; that is, a pulse or burst of RF energy is transmitted, with the received signal sampled once or a number of times. Each successive sample corresponds to a more distant range cell. The depth of the detection zone is approximately equal to the length of the pulse multiplied by the speed of light. For example, a 200 nsec pulse can yield a range resolution of 60 m or better. As a result, fine resolution in range requires a short pulse and therefore a higher peak power. In an effort to reduce the peak power requirement while still maintaining the same range resolution, radar designers utilize pulse compression signals. The more common pulse compression signals used are the frequency chirp and phase-coded waveforms. A chirp waveform is accomplished by a gradual (or step-wise) increase or decrease in the rate of change of phase of the transmitted signal. Phase-coded signals are obtained by changing the phase, at instants determined by a code sequence, in a smooth or abrupt fashion of an otherwise continuous wave signal. There are a variety of code sequences that may be suitable for radar pulse compression. Pseudonoise (PN) sequences, Barker codes and complementary series are examples of code sequences that have favorable characteristics in terms of radar detection. The means to compress the returned pulse compression signal so as to achieve the same integrated response as a single pulse having the same total duration as the pulse compression signal (without sacrificing range resolution) may also take a variety of forms. Some of the more common techniques and technologies include digital correlation, active correlation, SAW devices, CCD correlators, and acousto-optic devices.
A line sensor using pseudo-random codes is disclosed in U.S. Pat. No. 4,605,922. This patent teaches a microwave motion sensor system using spaced transmitting and receiving antennas. The transmitted signal is modulated by a pseudorandom code to cause a spreading of the transmitted signal over a wide frequency band. This renders any jamming techniques ineffective. The receiver has a similar pseudo random code generator to that in the transmitter and locks on to the transmitted code. The random code sequence is not used for range gating as is done in the invention of this application.
U.S. Pat. No. 4,458,240, issued July 3, 1984 (issued on a divisional application of U.S. Pat. No. 4,187,501) shows a system using transmission line sensors in which the starting phase of the transmitted signal is switched by 0.degree. or 180.degree. from pulse to pulse under the control of a pseudo-random code generator. As in U.S. Pat. No. 4,605,912, this spreads the spectral energy and greatly reduces the effect of any interfering signal. The random code sequence is not used for range gating as it is in the invention of the present application.
A Continuous wave (CW) or multi-CW radar, though capable of providing a simple receiver design because of a high transmit signal duty-cycle at or near unity, cannot satisfy many of the above requirements. The lower bound on the sampling rate for a CW radar, given by the Nyquist sampling theorem, can be as low as a few tens of Hertz for the targets of interest. Movement around the antennas can overwhelm the return signal from targets just a few tens of meters away since there is no range-gating capability with such a signal. Similarly, large targets which are past the desired detection area cannot be suppressed; consequently, railways and roadways near the desired detection range can result in an unacceptably high nuisance alarm rate. Because of the inability to provide range-gating with CW signals, the nonfluctuating portion of the received signal, also referred to as the profile or stationary clutter, is usually quite large, often placing limits on the receiver sensitivity. U.S. Pat. No. 4,595,924 describes a Very High Frequency (VHF) CW Doppler radar.
A more conventional pulse-type radar, while capable of providing one or more "range cells", has numerous drawbacks. These include a much faster, more complex and therefor more costly data collection process, susceptibility to intentional or unintentional interference, ease of targeting by hostile forces, and an increased peak transmit power because of the transmit signal's low duty-cycle (typically well under 10%). As well, sampling in excess of 10 MHz is required for a detection zone resolution of 30 m or less. U.S. Pat. No. 3,603,996 describes such a pulsed Doppler radar.